Process for preparing an optically active succinimide derivative, one of the key intermediate for ranirestat, is disdosed in JP 1994-192222A and J. Med. Chem., 41, 4118 (1998). The process is shown in the scheme below, wherein R is a benzyloxycarbonylamino group or a pyrrol-1-yl group:

A sole process for preparing an optically active intermediate of ranirestat disclosed in JP1994-192222A and J. Med. Chem., 41, 4118 (1998) is an optical resolution method, in which a salt of racemic succinimide derivative with cinchonidine is crystallized.
JP1993-186472A discloses a process for preparing ranirestat shown in the scheme below,
wherein R is a (−)-menthyl group.
In the process above, the optically active intermediate (a compound of Reference Example 16 in JP1993-186472A) was prepared from ethyl (−)-menthyl 2-(pyrrol-1-yl)malonate (a compound of Reference Example 2 in the said patent literature) through three steps. Each compound of the Reference Example 2 (oily product), the Reference Example 9 (oily product) and the Reference Example 12 (crystalline product) in JP1993-186472A is a diastereomeric mixture having (−)-menthyl ester as a partial structure, but all the (−)-menthyl ester are not always crystalline products or separation of the diastereomer is not always successful even if a crystalline product was obtained.
In the above patent literature for example, a diastereomer of (−)-menthyl ester is not isolated except the compound of Reference Example 16.